X-Rays Abound When Lightning Strikes

Courtesy of the University of Florida, Lightning Research Laboratory
Lightning appears to be nature's own particle accelerator after all. In the 1920s, Nobel prize-winner C. T. R. Wilson suggested that lightning could produce electrons traveling at the speed of light. Ever since then, scientists have been presenting evidence for and against that notion, based on observations of naturally occurring lightning flashes and satellite images of x-rays surrounding thunderheads. Many investigators believed that the lower atmosphere was too dense for electrons to accelerate to speeds high enough to emit x-rays and other high-energy particles. Instead, they thought that lightning worked by conventional energy discharge--a bigger version of the spark that occurs when you touch a doorknob after trudging across the rug. Now researchers have measured short bursts of intense radiation during controlled lightning strikes. The results suggest that the electric fields associated with lightning storms are strong enough to overcome the drag forces experienced by electrons as they collide with air molecules in the dense lower atmosphere, thereby allowing them to zip around at relativistic speeds.

Nature does not plan lightning storms around the location and time schedules of interested scientists, which made it difficult for early workers to study lightning-induced radiation. Joseph Dwyer of the Florida Institute of Technology and Martin Uman of the International Center for Lightning Research and Testing at the University of Florida circumvented this problem by triggering lightning during storms, a common practice among modern lightning researchers. To do this, the team launched a rocket tethered to the ground by a copper wire into a thunderhead, aiming to corral lightning bolts as they struck the rocket and followed the wire to the launch pad. They succeeded in harnessing 37 individual lightning strokes, and found intense bursts of x-ray radiation--each typically depositing tens of megaelectron volts into the detector--associated with 84 percent of them.

"How lightning works really needs to be revisited," Dwyer asserts. Several years ago, scientists working atop a mountain in New Mexico measured x-rays from naturally occurring lightning, but their experiment could not be replicated. The new work, detailed today in the journal Science, is the first controlled and repeatable experiment showing that lightning emits energetic radiation. Says Uman, "Our measurements with the controlled lightning will probably lead others to believe that the mountain top measurement was right."